1. Field of the Invention
The present invention relates to a dephlegmator which is mainly used as an alternative to a distillation apparatus in a low-temperature separation plant such as a natural gas liquefaction plant.
2. Description of the Related Art
A dephlegmator is provided with feed gas passages and refrigerant passages. A feed gas introduced into the feed gas passages from the bottom is partially condensed by a refrigerant that passes through the refrigerant passages, components having low boiling points are withdrawn as rectified gases from the top, and components having high boiling points are discharged as condensates from the bottom. This process is known as "rectification".
The dephlegmator is generally provided with a plate-fin exchanger in which passages are formed by alternately stacking corrugated heat-transfer fins and pass partition plates.
FIG. 9 is a partial sectional view of a core (a block of stacked heat-transfer fins and pass partition plates) C in the plate-fin exchanger, and FIG. 10 shows the passage arrangement of the core C.
In the drawing, numeral 1 represents a heat-transfer fin, numeral 2 represents a pass partition plate, and numeral 3 represents a side bar. A refrigerant passage 4 and a feed gas passage 5 are alternately formed between the individual pass partition plates 2.
A feed gas A flows, for example, from the bottom to the top through the feed gas passages 5, and a portion of the feed gas is condensed by means of heat exchange with a liquid or gas-liquid two-phase refrigerant B which passes through the refrigerant passages 4. A condensate AL is discharged from the bottom and a gas fraction (rectified gas) is discharged from the top.
In the plate-fin exchanger type dephlegmator, the following two structures are known with respect to a relationship between the flow direction of the refrigerant and that of the feed gas:
1. a structure, as shown in FIG. 11, in which the refrigerant flows downward from the top to the bottom of the core C and the feed gas flows upward from the bottom to the top, i.e., countercurrently (for example, refer to U.S. Pat. No. 4,002,042; hereinafter referred to as conventional art 1) PA1 2. a structure, as shown in FIG. 12, in which the refrigerant flows upward from the middle section of the core C, in the same direction as that of the feed gas, i.e., in a parallel flow state (hereinafter referred to as conventional art 2)
The conventional arts 1 and 2, however, have drawbacks as described below.